The present invention relates to a frequency generator; that is, to a method and apparatus for generating a clock signal having an accurate and stable frequency which is adjustable, over a given range, in small (e.g. 0.05 Hz) programmable steps.
Many techniques are known for generating electrical signals with a prescribed frequency. The general theory of frequency generation is described in the text Frequency Synthesizers: Theory and Design by Vadim Manassewitsch, published by John Wiley & Sons. 1987. Specific circuits which represent the state of the art are disclosed in the U.S. Pat. Nos. 4,494,073; 4,603,217; 4,558,282; 4,458,329 and 3,982,199.
Traditionally, a clock signal with an accurate and stable frequency is derived from a crystal controlled oscillator. If only a single frequency is desired, this frequency may be obtained by choosing a relatively high oscillator frequency and then dividing this frequency by an integer number N. If a 50% duty cycle is desired, the frequency must be divided by an even number N.
If an adjustable frequency is desired, this technique is inconvenient. Even if the number N can be any integer, step changes in frequency due to increments in the number become smaller and smaller in a non-liner fashion as the number increases. That is: ##EQU1## As may be readily seen from a plot of the step increment S against the dividing number N, the increment S is extremely non-linear for small values of N but approaches linearity with increasing values of N. In many applications, the original frequency F can be selected high enough such that, after dividing by the number N, the largest step increment S still provides the desired resolution in the frequency range of interest.
However, as this frequency range is expanded and must itself include relatively high frequencies, this technique of "dividing down" has proved inadequate.
To meet the requirements of the rapidly growing field of communications, a new family of approaches and techniques have emerged. The most commonly used techniques are those involving pulse stealing, fractional division, frequency mixing and the use of one or more phase locked loops. Such techniques are described in the aforementioned text Frequency Synthesis: Theory and Design. However, the pulse stealing and fractional division techniques result in frequency jitter around the stealing pulse frequency. With the phase locked loop technique, it is impossible to neglect the phase and filter noise on frequency accuracy and stability.
There is therefore a need to provide a frequency generator which produces an accurate, stable frequency which is adjustable in small, equal increments over a relatively large range of frequencies.